Patch "hugetlb: unshare some PMDs when splitting VMAs" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    hugetlb: unshare some PMDs when splitting VMAs

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     hugetlb-unshare-some-pmds-when-splitting-vmas.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 8b58f4839634ed814a53f71c0831a2e4fa0e9292
Author: James Houghton <jthoughton@xxxxxxxxxx>
Date:   Wed Jan 4 23:19:10 2023 +0000

    hugetlb: unshare some PMDs when splitting VMAs
    
    [ Upstream commit b30c14cd61025eeea2f2e8569606cd167ba9ad2d ]
    
    PMD sharing can only be done in PUD_SIZE-aligned pieces of VMAs; however,
    it is possible that HugeTLB VMAs are split without unsharing the PMDs
    first.
    
    Without this fix, it is possible to hit the uffd-wp-related WARN_ON_ONCE
    in hugetlb_change_protection [1].  The key there is that
    hugetlb_unshare_all_pmds will not attempt to unshare PMDs in
    non-PUD_SIZE-aligned sections of the VMA.
    
    It might seem ideal to unshare in hugetlb_vm_op_open, but we need to
    unshare in both the new and old VMAs, so unsharing in hugetlb_vm_op_split
    seems natural.
    
    [1]: https://lore.kernel.org/linux-mm/CADrL8HVeOkj0QH5VZZbRzybNE8CG-tEGFshnA+bG9nMgcWtBSg@xxxxxxxxxxxxxx/
    
    Link: https://lkml.kernel.org/r/20230104231910.1464197-1-jthoughton@xxxxxxxxxx
    Fixes: 6dfeaff93be1 ("hugetlb/userfaultfd: unshare all pmds for hugetlbfs when register wp")
    Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
    Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
    Acked-by: Peter Xu <peterx@xxxxxxxxxx>
    Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
    Cc: Muchun Song <songmuchun@xxxxxxxxxxxxx>
    Cc: <stable@xxxxxxxxxxxxxxx>
    Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index e7bd42f23667..8599f16d4aa4 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -82,6 +82,8 @@ struct mutex *hugetlb_fault_mutex_table ____cacheline_aligned_in_smp;
 
 /* Forward declaration */
 static int hugetlb_acct_memory(struct hstate *h, long delta);
+static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
+		unsigned long start, unsigned long end);
 
 static inline bool subpool_is_free(struct hugepage_subpool *spool)
 {
@@ -4164,6 +4166,25 @@ static int hugetlb_vm_op_split(struct vm_area_struct *vma, unsigned long addr)
 {
 	if (addr & ~(huge_page_mask(hstate_vma(vma))))
 		return -EINVAL;
+
+	/*
+	 * PMD sharing is only possible for PUD_SIZE-aligned address ranges
+	 * in HugeTLB VMAs. If we will lose PUD_SIZE alignment due to this
+	 * split, unshare PMDs in the PUD_SIZE interval surrounding addr now.
+	 */
+	if (addr & ~PUD_MASK) {
+		/*
+		 * hugetlb_vm_op_split is called right before we attempt to
+		 * split the VMA. We will need to unshare PMDs in the old and
+		 * new VMAs, so let's unshare before we split.
+		 */
+		unsigned long floor = addr & PUD_MASK;
+		unsigned long ceil = floor + PUD_SIZE;
+
+		if (floor >= vma->vm_start && ceil <= vma->vm_end)
+			hugetlb_unshare_pmds(vma, floor, ceil);
+	}
+
 	return 0;
 }
 
@@ -6349,26 +6370,21 @@ void move_hugetlb_state(struct page *oldpage, struct page *newpage, int reason)
 	}
 }
 
-/*
- * This function will unconditionally remove all the shared pmd pgtable entries
- * within the specific vma for a hugetlbfs memory range.
- */
-void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
+static void hugetlb_unshare_pmds(struct vm_area_struct *vma,
+				   unsigned long start,
+				   unsigned long end)
 {
 	struct hstate *h = hstate_vma(vma);
 	unsigned long sz = huge_page_size(h);
 	struct mm_struct *mm = vma->vm_mm;
 	struct mmu_notifier_range range;
-	unsigned long address, start, end;
+	unsigned long address;
 	spinlock_t *ptl;
 	pte_t *ptep;
 
 	if (!(vma->vm_flags & VM_MAYSHARE))
 		return;
 
-	start = ALIGN(vma->vm_start, PUD_SIZE);
-	end = ALIGN_DOWN(vma->vm_end, PUD_SIZE);
-
 	if (start >= end)
 		return;
 
@@ -6400,6 +6416,16 @@ void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
 	mmu_notifier_invalidate_range_end(&range);
 }
 
+/*
+ * This function will unconditionally remove all the shared pmd pgtable entries
+ * within the specific vma for a hugetlbfs memory range.
+ */
+void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
+{
+	hugetlb_unshare_pmds(vma, ALIGN(vma->vm_start, PUD_SIZE),
+			ALIGN_DOWN(vma->vm_end, PUD_SIZE));
+}
+
 #ifdef CONFIG_CMA
 static bool cma_reserve_called __initdata;